nixos/sssd-ldap: fix eval

Introduced by fd7d901133. The openldap
module now expects the database directory to be below
`/var/lib/openldap`, oterhwise it'll fail evaluation like this:

    Failed assertions:
    - Database dc=example,dc=org has `olcDbDirectory` (/var/db/openldap) that is not a subdirectory of
    `/var/lib/openldap/`.

nixos/tests/sssd-ldap.nix

@@ -28,7 +28,7 @@ in import ./make-test-python.nix ({pkgs, ...}: {
             attrs = {
               objectClass = [ "olcDatabaseConfig" "olcMdbConfig" ];
               olcDatabase = "{1}mdb";
-              olcDbDirectory = "/var/db/openldap";
+              olcDbDirectory = "/var/lib/openldap/db";
               olcSuffix = dbSuffix;
               olcRootDN = "cn=${ldapRootUser},${dbSuffix}";
               olcRootPW = ldapRootPassword;